The present invention generally relates to method and apparatus for automatically loading powder material into a mold and, more particularly, to such method and apparatus for automatically loading a desired amount of powder material into a mold which comprises a tubular body having a bore extending therethrough. The mold may be either a sintering mold used for sintering the powder material loaded therein during sintering process or a powder-compact-forming mold used only for forming a powder compact therein while the powder compact thus formed is subjected to sintering process after being removed from the mold.
There have been provided various powder material loading apparatus for loading an amount of powder material into a sintering mold to form a powder compact in the mold, which is retained in the mold during a subsequent sintering process, such as an electrical sintering process. However, there has not been proposed an idea of a continuous fabrication process for obtaining sintered products, including the steps of: loading an amount of powder material into a sintering mold in the form of a plurality of layers; heating the powder compact retained in the mold for sintering; and removing the sintered product from the mold. This idea has not been proposed primarily because conventional electrical sintering techniques require a relatively long time to complete the sintering process. In consequence thereof, any of conventional automatic powder material loading apparatus was not intended for such a continuous fabrication process but provides solely the function for loading powder material into a mold.
Recently, many improvements has been made in electrical sintering methods. For example, Pulsed Current Energizing Sintering (or Pulsed Electric Current Sintering) method using a pulsed current and including Spark-Plasma Sintering, Electric-Discharge Sintering and Plasma-Activated Sintering methods proposed by the applicant of this application has been improved. According to the improved Pulsed Current Energizing Sintering, sintering time is drastically shortened. Such shorter sintering time provides the possibility of realizing a continuous fabrication process for obtaining sintered products, including the above mentioned steps. Therefore, there have now arisen demands for such a method and apparatus for loading powder material into a mold that may be suitably used for such a continuous fabrication process.
In addition, by virtue of newer electrical sintering techniques, such as those mentioned above, such materials that were difficult to bond together through any older techniques can be now bonded together with ease into a unitary sintered product. Examples of such materials are: a stainless steel vs. copper; a ceramic vs. a metal; etc. Such a unitary sintered product of two different powder materials may be fabricated to have two-layered structure composed of two layers bonded together and each made of a pure powder material; however, the characteristics of such a sintered product can be improved by adding at least one middle layer to create such multi-layered structure in that the middle layer is made of a mixture of the two powder materials. Further, such multi-layered structure may be also used with advantageous for a sintered product including three or more layers made of respective powder materials which are identical in composition and differ from one another only in particle size, wherein the powder materials for the layers have their particle sizes gradually increasing from the layer on one side of the product toward the other side. Such a sintered product may have gradient functionality (i.e., the gradual variation in properties of the sintered product from one side of the product to the other) so as to achieve more improved characteristics. In order to fabricate a sintered product having gradient functionality, it is required to load different powder materials, which differ from one another in at least one of properties including component(s) of powder material, percentages of components, particle size and particle shape, into a mold so as to form corresponding powder layers of desired thickness with precision. While there have been proposed various automatic powder material loading apparatus, none of them is capable of loading different powder materials into a mold to form a multi-layered powder compact in a fully automated manner. In addition, in order to fabricate high-quality sintered products having gradient functionality with good reproducibility, not only the capability of forming a multi-layered powder compact but also other various capabilities are required, so it is the case that none of conventional powder material loading apparatus is suitable for fabrication of sintered products having gradient functionality.